1. Field of the Invention
This invention concerns concentrated styryl dye solutions comprising dye, acetic acid and/or propionic acid, acrylic acid and/or methacrylic acid, and water.
2. Description of the Prior Art
Most paper is colored by the addition of a dye to an aqueous slurry of the paper pulp, for example in the beater, during manufacture of the paper. U.S. Pat. No. 3,742,012 discloses basic, quaternary ammonium styryl dyes (including the styryl dye of the solutions of this invention). These dyes are of significant commercial importance since they impart bright, green-yellow shades to paper, possess good tinctorial strength and substantivity and are readily and completely removed from the paper by conventional bleaching agents. U.S. Pat. No. 3,742,012 does not suggest the dye solutions of this invention.
Attempts have been made to prepare stable, concentrated solutions of basic dyes. These attempts have included the conversion of basic dyes to water-soluble salts of carboxylic acids, and the subsequent dissolution of the salt in a water-miscible organic solvent as disclosed, for example, in U.S. Pat. No. 3,346,322. Attempts have also included use of aqueous media; use of a variety of water-soluble solvents and surfactants and combinations of the above. In the instant case, however, all of these approaches have inherent disadvantages. For example, styryl dye precipitation occurs from aqueous carboxylic acid solutions upon storage at low temperature (0.degree. C). This results in a dye solution of nonuniform strength and causes undesirable specking of the paper during the dyeing process. In aqueous media, large quantities of water are required to keep the dye in solution. Water, however, causes dye hydrolysis and is responsible for strength loss observed during prolonged storage at temperatures above 25.degree. C.
There is a need, then, for provision of basic, styryl dyes, such as the dye described herein, in a concentrated solution form which can be easily handled and added to the pulp slurry in accurately measured amounts by liquid pumping and metering means. Also, a solution offers the further advantage of convenience in shipping and handling since the problem of dusting commonly encountered with the powdered dyes is eliminated. To be practical, however, the solution should contain a rather high concentration of dye. Also, it must have stability, both chemical and physical, upon storage for extended periods of time even under the adverse conditions of high and low temperatures which are sometimes encountered. Thus, for the solution to be most useful, there should be no dye decomposition or settling out on storage and no appreciable hydrolysis under conditions of storage and use.